Discrete Isolation and Continuous Area Drive Genetic Divergence Across Mediterranean Islands
Leonardo Dapporto, Claudia Bruschini, Adele Bordoni, Alessandro Cini, Vlad Dincă, Luca Di Nuzzo, Alessio Iannucci, Mattia Menchetti, Lorenzo Pasquali, Vania Salvati, Loukia Spilani, Gerard Talavera, Chiara Vergata, Raluca Vodă, Sara Fratini, Roger VilaABSTRACT
Aim
Comparison between islands and equivalent mainland areas to dissect the effect of area, isolation and species traits in determining island genetic endemicity and genetic differentiation.
Location
The Western Mediterranean region.
Time Period
Current.
Major Taxa Studied
Butterflies (Lepidoptera, Papilionoidea).
Methods
We analysed 10,367 COI sequences from 105 species, 34 islands and 47 sea straits, along with four functional species traits. We compared determinants of genetic diversity (nucleotide diversity, NucDiv ), endemic mutations (EM) and the Dst fixation index between island populations and similarly sized mainland populations (Continental Area Equivalents, CAEs). Generalised linear mixed models tested fixed effects and interactions of island characteristics and species traits. We also evaluated whether population‐level effects (individual species increasing differentiation with island size and isolation) or community‐level effects (larger, less isolated islands hosting more genetically divergent species) better explain observed island patterns.
Results
CAEs and islands exhibited highly distinct genetic signatures. NucDiv was higher in CAEs while Dst was higher across sea straits than between land areas separating CAEs, particularly for larger islands and non‐migratory species. EM increased with island area, especially in non‐migratory and small species. Island communities were significantly nested. Widespread species also showed lower genetic variation, thus producing a strong community‐level effect. The population‐level effects showed a weaker effect. Counterintuitively, increasing isolation did not increase divergence, as both processes tended to reduce genetic differentiation in more isolated islands.
Main Conclusions
Comparing islands with CAEs reveals that even narrow sea barriers of a few kilometres greatly reduce gene flow in non‐migratory butterflies, increasing genetic differentiation to a greater extent than similar mainland distances. After controlling for the presence of sea straits, island size predicts genetic divergence while increasing isolation does not. Overall, endemicity remains low, indicating that mainland‐driven genetic turnover, rather than in situ evolution, dominates insular genetic patterns.